The present invention relates to an image reproduction method by joining adjacent images, such as two or more image bands, in an image generating device, such as a printer.
Nowadays many types of image generating devices exist, including phototypesetters, imagesetters, lithographic printers and electronic printers for printing electronic (colour) images. The generated images may be e.g. latent, visible or lithographic and are generated on a suitable image carrier. An image carrier may be paper, a transparent PET (polyethyleenterephtalate) material, photographic material, an electrophotographic drum or a lithographic printing plate etc. A non-visible image usually undergoes a process to generate a visible image from it: a latent image may be developed; a lithographic image, comprising ink accepting and ink repellent zones, may be provided with ink, which is transferred to a paper image carrier to render a visible image.
Some printers use thermal processes to form an image. These may be direct thermal systems, thermal (wax) transfer system or thermal systems using dye sublimation to form images on a receiving material or image carrier. The thermal process can be activated by using a thermal head or infrared (IR) light sources. An IR light source commonly used in laser thermal printers is a semiconductor laser. Other popular printing systems use an ink jet printing technology. Droplets of fluid ink are ejected to a receiving layer or image carrier to form a visible image.
A very common type of printer in the office environment is a printer using an electrographic process. According to the electrophotographic process, which is a specific electrographic process, a latent electrostatic image is formed by selectively illuminating or exposing an electrostatically charged photoconductive drum and developing the latent image by toner, thereby producing a visual toner image. The toner may thereafter be transferred to an image carrier or substrate made of e.g. paper. Another electrographic process is referred to as Direct Electrographic Printing (DEP) and is described in e.g. EP-A-0 675 417. According to this technique, a toner cloud is brought in the vicinity of a print head structure. That structure has apertures that may be xe2x80x9copened or closedxe2x80x9d by electrostatic action. By image-wise opening these apertures, toner particles travel image-wise through the apertures of the print head structure and impinge on an image substrate such as an intermediate image drum or a final image substrate such as paper or a transparency material. Most of the above mentioned printers use print heads: these are units carrying the image-forming or image-generating elements, and which e.g. provide the heath, emit light or eject ink or toner particles in an image wise fashion.
A print head is generally not capable to generate at once the complete image on the image carrier. For generating an image, the area of the image carrier is traditionally (mentally) partitioned in tiny addressable units, referred to as microdots. These microdots are disjunctive, i.e. they do not overlap each other, and all the microdots together fill the complete image area on the carrier. As such, they form a real partition of the image carrier. The microdots may be obtained by a grid defined by a first set of parallel equidistant lines having a first orientation and a second set of equidistant parallel lines having a second orientation different from the first orientation. The tiny parallelogram areas, enclosed by two sets of two parallel line portions, are referred to as microdots. If the second orientation is orthogonal to the first orientation, then the microdots have a rectangular shape. If the distance between two consecutive lines of the first set equals to such distance of the second set, then the microdots have a rhombic shape. In most cases the orientation is orthogonal and the distance is identical, resulting in square microdots. The multiplicative inverse of the size of the side of a square microdot is referred to as the spatial resolution of the imaging device. The microdot size in an electrophotographic device may be 42 micron (xcexcm). The spatial resolution of the device is then 1000/42 microdots per mm, i.e. 24 dots per mm or 600 dots per inch (600 dpi). The notion of an xe2x80x9caddressablexe2x80x9d microdot refers to the fact that the imaging device is capable to address the microdot individually. A binary electrographic device is capable to deposit either a maximum amount of toner or a minimum amount of toner on each individual microdot. Although the microdots are disjunctive, is may be possible that some toner particles designated for a first microdot, also partly cover an adjacent microdot, i.e. a microdot that has a side or a corner in common with the first microdot. Examples of such binary devices are the Agfa P400, P3400 and P3400PS devices, developed and marketed by Agfa-Gevaert N.V. in Mortsel, Belgium and having a resolution of 400 dpi. A multilevel electrographic device is capable to deposit on each individual microdot specific variable amounts of toner, expressed in microgram per square millimeter (xcexcg/mm2) . The number of such specific amounts may be e.g. 16, such as in the Chromapress system, developed and marketed by Agfa-Gevaert N.V. in Mortsel. The lowest amount of toner may be generated by offering to the electrophotographic system a digital value 0, whereas the highest amount of toner may be generated by offering to the electrophotographic system a digital value 15. All values between 0 and 15 may generate on the individually addressed microdot each a specific amount of toner between said minimum and maximum amount. Since values from 0 to 15 may be represented by four bits, this system is referred to as a 4-bit multilevel system. To achieve the impression of continuous tone for images reproduced on such system, it may be necessary to introduce some form of halftoning, as described in EP-A-0 680 195, EP-A-0 634 862 and EP-A-0 682 438. It follows that an A4-sized image carrier (297 mmxc3x97210 mm) comprises about 35 million microdots in a 600 dpi (ca. 24 microdots per mm) system. An imagesetter for generating a printable image may have a spatial resolution of 2400 dpi (ca. 95 microdots per mm). If the film or printing plate has a size of 14xe2x80x3xc3x9717xe2x80x3 (14 inch by 17 inch, i.e. 356 mm by 432 mm), the number of microdots on the image carrier amounts to 1,371 million. An imaging device capable to address that large number of microdots at once and at that resolution would be too costly. Therefore, the image carrier is rather exposed line by line, e.g. by using a LED array or even pixel by pixelxe2x80x94i.e. microdot by microdotxe2x80x94by using a sweeping laser beam. A sweeping laser beam may be generated by an imaging device comprising a laser source and a light deflection system such as a rotating polygonal mirror or a rotating pentaprism. In more complex systems, all microdots arranged on a plurality of lines may be addressed at once, i.e. at the same instant. This may be achieved by a plurality of parallel LED arrays in an electrophotographic system and by a print head structure having a plurality of parallel lines of printing apertures in a DEP device. A plurality of sweeping laser beams may give the imaging device the capability to address a plurality of microdots at once. According to the above mentioned systems, one imaging devicexe2x80x94i.e. one LED array, one printhead structure, or one laser beam system comprising a laser source and a deflection meansxe2x80x94is capable to address one line of microdots simultaneously or at least within a short period of time. With that short period is meant the time to address all the microdots of one line, without addressing within that period other lines by the same imaging device.
Due to the cost of some complex devices, it is sometimes too expensive to provide a print head having a length equal or larger than the width of the recording material. For some technologies it is even impossible to make a good quality print head of a large size. As such, the shorter print head cannot address instantly all microdots arranged on one line running from one side to the opposite side of the image carrier.
Especially when printing a large size image, e.g. posters, the print head can print only a portion of the image. A poster may have a size of 1.5 m width and 2.5 m length. In a 75 dpi system, state of the art systems have a printing head with a width of 30 cm. To cope with a poster width of 150 cm, the printing head has to make at least five steps. Therefore the image is printed in several parallel bands, referred to as sub-images, which are sequentially printed alongside each other.
In an inkjet printer having an array of nozzles arranged in a longitudinal direction parallel to the longer side of the paper to be printed, the paper is fed stepwise relative to the print head in a longitudinal direction. The print head has a transversal shuttle movement relative to the paper for printing image bands by simultaneous operation of the plurality of nozzles. The bands are printed one after another. A first image band or sub-image is printed during a first transversal shuttle movement. Thereafter, the paper is moved stepwise in a first longitudinal movement. Then the second sub-image is printed during a second transversal shuttle movement, followed by a second longitudinal stepwise movement etc. In a thermal laser transfer printer the imaging material can be mounted on a drum. While the drum is rotated the print head is stepwise moved along the rotation axis printing the image band sequentially alongside each other. One such printer is described in WO 93/04 552 where a thermal print head carrying diode lasers coupled to fibres is displaced alongside the rotatable drum. Sequentially printing the image in bands or sub-images may give the following problems.
1. When image bands do not exactly join together it is possible that a distinct white line in between the printed bands becomes clearly visible as an image defect. On the other hand, when the image bands overlap, a clearly visible dark line disturbs the image.
2. Even when print bands join perfectly along the length, a slight mismatch in the position of the bands along this length can cause visible artefacts.
As can be seen in FIG. 2, the mismatch due to displacement of a first sub-image 21 relative to a second sub-image 22 according to the size of only one microdot can cause a visible defect when printing screened (binary) images. This is referred to as a phase defect of the screened data.
The artefacts, caused in the image zone 26 where two sub-images join, may find their origin in the imperfect placement of the printing head for printing the second sub-image in relation to the first printed sub-image. This may be due to play of the mounting and moving system of the printing head.
The very accurate positioning systems, needed to solve the above problem, are too expensive to install in the printers destined to the consumer market.
The above problems have already been recognised by other researchers and several solutions have been proposed.
In EP-A-0 522 980 and EP-A-0 619 188 there is proposed to make an overlap zone of two printed bands in a thermal sublimation printer where two fitting stochastic rasters gradually fade towards the neighbouring band.
In a laser thermal transfer proofer described in EP-A-0 529 535 and WO 93/4 552 the outermost lines of each band are so called xe2x80x9cdummyxe2x80x9d lines. The information recorded in these lines has the purpose to avoid the occurrence of white side lines due to incorrect placement. In DE-A-4 110 776 the joining of the bands in an ink jet printer using a shuttling print head is not done along a straight line but along a curved (random) path. Despite of all the proposed measurements hitherto, there is still a need to obtain a good quality joining of printed bands.
It is an object of the invention to provide a method for the reproduction of an original image including high quality joining of two sub-images that are printed sequentially by one imaging device or that are printed by two different imaging devices.
The above mentioned objects are realised by a method having the specific features defined in claim 1. Specific features for preferred embodiments of the invention are set out in the dependent claims.
The original image may be an image of a real scene, captured e.g. by a photographic camera or a digital camera. The original image may also be an image on black and white or colour print material. Such an image may be converted to an electronic image by an image scanner, such as the Agfa SelectScan(trademark) digital scanner. Such electronic image may also be designated as an original image. An original image may also originate from a software application such as PhotoShop (Trademark of Adobe Inc.), for creation or modification of original or synthetic images. An original electronic image may represent a black and white image or a colour image. An electronic image is traditionally represented as one or more rectangular matrices of image pixels, wherein each pixel is represented by a digital value. The digital value typically ranges from 0 to 255, where 0 may represent dark and 255 may represent light or vice versa. Colour images are usually represented by three matrices, each matrix representing a colour component such as red, green, blue; or cyan, magenta, yellow; or hue, intensity, saturation; etc. Where the current invention refers to an original image, the following may i.a. be referred to: the electronic representation of a black and white continuous tone image, of one colour component of a continuous tone image.
An image carrier is e.g. paper, transparent or opaque film material, such as PET, etc. on which the reproduction is made visible. Before the image is visible, several intermediate operations may be needed, such as developing a latent image, applying ink to a lithographic medium and printing on paper, etc.
A sub-image is an image that is a portion of the original image or a derivative thereof. If an electronic image comprises 512 pixels on 512 lines, then a first sub-image may comprise 300 pixels on 512 lines, e.g. pixels 1 to 300 for each line of the original image and a second sub-image may also comprise 300 pixels on 512 lines, e.g. pixels 213 to 512 for each line of the original image.
According to the current invention, the first and second sub-image are conjoined, i.e. they have a common region. According to the above example, the first and second sub-images have pixels 213 to 300, i.e. 88 pixels, for each line in common, i.e. 88*512=45,056 pixels. The first and second sub-image must be representative for a portion of the original image, i.e. it is possible to reproduce at least a portion of the original image on an image carrier by the first and the second sub-image.
The overlap region is defined as a region on the final carrier, i.e. where the reproduction of the original image is visible. In the overlap region, both the first sub-image and the second sub-image give a contribution to the integral optical density of the image carrier. By image carrier is meant the substrate of the carrier (e.g. paper, PET, . . . ) along with the toning agent, e.g. toner, ink, black silver (as opposed to e.g. white silver salt such a silver behenate, which is transformed to black silver by a thermal reaction), etc. Application of the toning agent to the substrate, changes the optical density of the image carrier at the location where the agent is applied. The optical density may be measured by a densitometer. According to the spot diameter, two types of densitometers may be distinguished: traditional densitometers, having a spot diameter of 3 mm typically, and 2 or 1 mm exceptionally; and, microdensitometers capable to measure the optical density of a spot having a diameter between 10 xcexcm and 400 xcexcm. The optical transmission density is measured by illuminating a transmissive material, e.g. the image carrier carrying an image, and measuring the transmitted light T. The 10-logarithm of the ratio of the incident light I and the transmitted light T is defined as the optical density:D=log10(I/T). For the definition of the optical reflection density, the reflected light R is used instead of the transmitted light T:D=log1xe2x80x83(I/R). For a large variety of imaging systems, the microdensitometer is capable to measure the density of one individual microdot. This density is referred to as microscopic density. If the microdensitometer is not capable to measure the microscopic density of one microdot, one may print a matrix of e.g. 20xc3x9720 identical microdots, and measure the microscopic density of the patch formed by the matrix. The traditional densitometer is not capable to differentiate individual microdots. If all microdots within the spot having a diameter of 3 mm have the same microscopic density, then the densitometer will read that density. If the microdots in that spot have different microscopic densities, the densitometer will read a mean value of these microscopic densities. This process is referred to as optical integration, and the measured optical density is referred to as integral optical density. This process corresponds also to what happens in the human eye, when it captures an image on an image carrier. Therefore, the visual interpretation of an image does not necessarily correspond to the microscopic density, but rather to the integral optical density. For the reproduction of an original image, it is more important that the integral optical density of the reproduction corresponds to the original image, rather than the microscopic density. To the human observer, a screened reproduction may look as pretty as a full continuous tone reproduction, although the microscopic densities of the screened reproduction do not match the integral optical density as observed. The integral optical density for an image consisting of a constant grey colour may also be defined by the mean microscopic density, taken over all microdots of a screen cell. A screen cell for a contone device corresponds to one microdot. For a screened image, a screen cell corresponds to the tile size of the screening method (see e.g. EP-A-0 682 438 for a definition of tiles).
A contribution to the integral optical density is defined as follows. Suppose that the first sub-image is printed alone, without printing second sub-image, on top of the first one. The integral optical density D1 of the image carrier with the first sub-image printed on it is measured, i.e. the final image carrier on which the reproduction is visible. This optical density D1 is the contribution of the first sub-image. By printing the second sub-image alone, the integral optical density D2 on the image carrier may be measured. This is the contribution by the second sub-image. The final optical density D in the overlap region, where both sub-image are imaged on top of each other, will generally obey the following inequalities, although exceptions are possible:
D1, D2xe2x89xa6Dxe2x89xa6D1+D2
For colour images, these contributions are measured per component, preferably by a colour densitometer. A colour densitometer is a densitometer including a specific colour filter, e.g. a red, green or blue colour filter. If an image is printed as a cyan, magenta and yellow component, then the contribution by the first sub-image for the yellow component is measured by printing the yellow component of the first sub-image alone, without any other component of the first sub-image, nor any component of the second sub-image. The integral optical density of the yellow component is then preferably measured by a colour densitometer using a blue filter.
A peripheral edge of a sub-image is one of the edges of the perimeter of the sub-image. If the sub-image is rectangular, then the sub-image has four peripheral edges. The peripheral edge in the overlap region, is the edge of the sub-image bordering the overlap region. Stricto sensu the first sub-image usually gives no contribution to microdots on the peripheral edge of the first sub-image, situated in the overlap region, but that that edge is also included in the overlap region.
Increasing the contribution by a sub-image is preferably realised by electronic image processing. This is set out below, mainly in conjunction with FIG. 5. If in an overlap region the contribution by the first sub-image increases, usually the contribution by the second sub-image decreases. The increase and decrease are such that the reproduction resembles the original image. Methods to achieve this are set out below.